Kirtan is part of an ancient form of Yoga known as Bhakti, or the Yoga of Devotion.
Kirtan is when a group of people come together to sing over and over various mantra. Mantra comes from two words, "man" meaning mind and "tra" meaning vehicle or instrument-to transport the mind from state of activity to one of stillness and peace. Kirtan is based on ancient mantras that have strong energetic and vibrational components and contain a transformative power and healing energy.

Kirtan is a very different kind of music. Based on ancient chants, it has the ability to quiet the mind if listened to with intention. Everyone experiences kirtan differently, and it doesn’t have to be a religious experience. You can think of it as a sing-along. A kirtan concert is not your typical concert either. Everyone sits on the floor, although chairs are usually available. The performers are accessible, in fact there’s not much of a distinction between performers & audience. The wallah (leader) sings the mantra, and the audience sings it back. A single chant can go on for up to forty minutes. As you sing with each other you experience a deep connection with the musicians, the other audience members and yourself. And when the music stops, your mind is quiet.
Because kirtan has roots in India, many of the songs are sung in Sanskrit. Some I choose for the New World Kirtan podcast are also in Punjabi, the language of the Sikhs. If you’ve ever chanted responses in Latin or Hebrew in your religious tradition, then you know how powerful singing in an ancient, holy language can be. You can be completely immersed in the sound, with no words to distract the mind. The magic of the chants can then carry you within.
Kirtan is non-denominational, the Universal language of Spirit, the song of the Soul.
( Ref: http://newworldkirtan.com/what-is-kirtan/ )
According to kirtan artist and bhakti yoga educator David Newman, kirtan means “to praise that which is exalted”—aka, the divine.
World-renowned mantra music artist Deva Premal puts it this way: “In kirtan, we sing our praises to the divine in the many forms in which it manifests.” Each chant revolves around a particular mantra"
What exactly is a mantra? And what’s a deity?
“People say, ‘I’m singing to the deities,’ but what does that mean? What is a deity?” says Grammy-nominated kirtan wallah Krishna Das. “It's like an older, deeper, bigger being. It's a space, a presence, a feeling. These names are the names of that place inside of us that is love, pure being, pure awareness, pure joy.” Kirtan—and other forms of mantra practice, such as seated meditation—help us uncover that place inside of us, he says: “our true nature.”
The literal translation of the word “mantra,” David says, is “to guide and protect the mind.” “But you could also say, ‘to guide the consciousness away from excessive thoughts.’ Mantra helps to guide your awareness to a place that is quiet and still,” he says.
The key to success in any form of mantra practice is repetition. When you’re sitting in a kirtan, gently bring your wandering mind back to the chant over and over again. With regular, sincere practice over time, says Krishna Das, you may notice that “thoughts don’t grab you so deeply. Emotions don’t wipe you out so completely. It changes your psyche.”
“Kirtan is the glue that bonds our hearts together.”
That’s a quote from David Newman, who observes that chanting as a group brings people together as a community in a way that no other form of yoga does. “I always feel very close to the people I’m chanting with, even if I’ve never met them before,” he says. In a world filled with messages about how we’re separate from each other and separate from the divine, chanting mantras together can provide an antidote. “Kirtan was created to fuel a sense of connection and unity,” he says.
( Ref: https://kripalu.org/resources/beginners-guide-kirtan-and-mantra )